Applicants claim, under 35 U.S.C. .sctn. 119, the benefit of priority of the filing date of Jun. 13, 1991, of a French application, copy attached, Ser. No. 9107215, filed on the aforementioned date, the entire contents of which are incorporated herein by reference.
Moisture testers are currently used for cereal grains, oleaginous and proteinaceous grains or other granular or powdered products. Known moisture measurement devices generally perform indirect (for example, hyperfrequency, infrared or dielectric) measurements, and among them, the capacitive moisture testers are quite satisfactory.
Capacitive moisture testers are based on the fact that the dielectric behavior of the analyzed product varies as a function of its water content. The measurement cell is embodied constitutes at least one capacitor, and the dielectric differences between the instant when the cell is empty and the instant when the cell is filled with the product, are measured.
These dielectric measurements may be made for each product sample at a constant volume (for example, the volume of the measurement cell) or a constant weight (sample having a predetermined weight, which can be dependent on the type of product). To make these measurements, the capacitor is for instance inserted into an oscillator, as will be described in further detail hereinafter.
However, moisture testers, even of the capacitive type, have their limits, among which one in particular is the difficulty of correcting the measurement error associated with the surface moisture of the product to be analyzed. In the course of a grain harvest, for example, if one batch is exposed to rain before it is tested, the measurement done at the receiving station is erroneous, because the surface moisture (free water) is not evaluated in the same way as the internal moisture in the grain (bound water). This is because, for capacitive measurement, the bound water has a much lower dielectric constant than the free water.
Attempts to correct the capacitive measurements have therefore been made, particularly by measuring dielectric capacitance at several frequencies, or by adding a measurement of resistivity to the capacitive measurement, and so forth. These known corrective measurements have certain disadvantages. In the case of measurements of resistivity or conductibility, those measurements depend on the nature of the salts contained in the water, which can invalidate the corrective measurements.